1. Field of the Invention
This invention relates to a musical tone signal generating apparatus of the type which generates a musical tone signal by sequentially and repeatedly reading a plurality of sampled data of the musical tone signal from a memory.
2. Prior Art
There has been proposed an electronic musical instrument having a musical tone signal generating apparatus of the type which generates a musical tone signal (or a musical tone waveform signal) by sequentially and repeatedly reading, from a memory, a plurality of data previously obtained by sampling the musical tone signal and stored in the memory. Such a musical tone signal generating apparatus is disclosed, for example, in U. S. Pat. No. 4,130,043. This conventional musical tone signal generating apparatus comprises a memory storing a plurality of sampled data which constitute one cycle of a given tone signal, a shift register having a plurality of stages which respectively store the plurality of sampled data read from the memory, and a digital filter circuit provided in a feedback path between output and input terminals of the shift register. The sampled data in the shift register are shifted by a clock signal having a frequency several times as high as a pitch of a tone corresponding to a key depressed on an associated keyboard. The tone signal is formed from the sampled data outputted from the last stage of the shift register. The sampled data sequentially outputted from the last stage of the shift register are also fed through the digital filter circuit back to the first stage of the shift register. As a result, the sampled data stored in each of the stages of the shift register varies at a rate determined by the filtering characteristic of the digital filter in accordance with the time elapsed. Thus, the musical tone signal formed from the sampled data outputted from the last stage of the shift register also varies with the lapse of time, so that the musical tone produced in accordance with the musical tone signal varies with time in tone color.
With such conventional musical tone generating apparatus, the shifting of the sampled data in the shift register is performed in accordance with the clock signal of which frequency is several times as high as the pitch of the tone corresponding to the depressed key of the keyboard. Therefore, the rate of variation of the value of each sampled data which is fed back to the shift register via the feedback path after filtration by the digital filter is substantially proportional to the frequency or pitch of the tone produced. As a result, the rate of change of tone color of a tone produced is proportional to a pitch (or a frequency) of the tone produced. With the conventional apparatus, it has therefore been impossible to give appropriate rates of change of tone color to musical tones corresponding to all the keys of the keyboard which covers several octaves. For example, the tone color of the musical tone of C4 note varies four times faster than that of the musical tone of C2 note. If the filtering characteristic of the digital filter is set to such a value that the tone color of C2 tone varies at an appropriate rate, the tone color of C4 tone will vary at an excessively high rate. On the other hand, if the filtering characteristic of the digital filter is set so that the tone color of C4 tone varies at an appropriate rate, the tone color of C2 tone will vary at an excessively low rate.